Development and application of molybdenum isotopes as a tool for tracking the evolving redox state of the Precambrian ocean

钼同位素的开发和应用作为追踪前寒武纪海洋氧化还原状态演变的工具

• 批准号: NE/D523235/1
• 负责人: Simon Poulton
• 金额: $ 20.45万
• 依托单位: University of Bristol
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2006
• 资助国家: 英国
• 起止时间: 2006 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FD523235%2F1
• 关键词: Development; application; molybdenum; isotopes; tool

Oxygen is vital to sustain many forms of life on Earth. Unlike the present-day, when life first evolved the atmosphere and oceans contained essentially no oxygen. Various lines of evidence suggest that the oxygen content of the atmosphere only began to rise about 2.3 billion years ago. Until recently it was thought that this also led to oxygenation of the ocean (as in the present day). A more recent hypothesis suggests that, instead, the increase in oxygen led to the weathering of sulphide minerals on land, which resulted in increased riverine delivery of sulphur to the oceans. The oceans then became rich in toxic hydrogen sulphide rather than oxygen (similar conditions are found in the modern day Black Sea). In fact, the ocean may only have become oxygenated following a second, much later rise in oxygen about 700 million years ago. All of this has profound consequences for the evolution of the biosphere. It is in the oceans where early life first evolved and flourished. The early biosphere was dominated by bacteria and the first photosynthesising bacteria probably evolved at least by 2.7 billion years ago, before the first major rise in oxygen. One of the puzzles of the early biosphere is why this early evolution of oxygen-producing photosynthesisers did not lead to the rapid oxygenation of the surface Earth thereafter. It is also clear that higher life forms, such as plants and animals (and ultimately humans) only began to evolve much later when the oceans eventually became oxic. Why was there a delay in the oxygenation of the surface Earth? Why did the biosphere only evolve slowly early in Earth's history? One prominent recent hypothesis attributes these puzzling features of the ancient Earth to ocean chemistry. One of the key requirements of photosynthesising bacteria is nutrients, which are essentially the elements contained in fertilisers- phosphorous, nitrogen, and trace metals such as molybdenum (Mo). Before the oxygenation of the atmosphere, the oceans were probably rich in dissolved iron (which is soluble in oxygen-poor water), leading to the widespread precipitation of chemical sediments very rich in iron (so-called Banded Iron Formations or BIFs). These may have taken vital nutrients like phosphorous and trace metals with them, leaving very low concentrations behind for bacteria to use. After the initial oxygenation of the atmosphere, and particularly if the oceans became sulphidic, trace metals may also have been in scarce supply as many of them are precipitated in the presence of hydrogen sulphide. This is important as it may have limited photosynthesis and hence oxygen production, helping to explain the apparent delayed oxidation of the Earth's surface, and hence the slow evolution of the biosphere. However, these ideas remain controversial. Detailed studies are required to assess whether the conditions described above did in fact exist. It is also important to determine how widespread these conditions were and how they affected nutrient availability. This project will examine nutrient availability as recorded by BIFs, the global extent of the transition to a sulphide-rich ocean following the first rise in atmospheric oxygen, and the chemical evolution of the oceans in the subsequent period of Earth's history leading up to the major explosion of animal and plant life. The tool we will use is the isotopes of molybdenum. The oceanic chemistry of Mo, and specifically the processes by which it is removed from solution into sediments, is highly dependent on the chemical state of the oceans. Further, these removal processes have variable preferences for the different isotopes of Mo, which makes the record of Mo isotope variations in the rocks interpretable in terms of both the oxygenation state of the ancient oceans, and the availability of Mo as a nutrient. This research should ultimately provide a better understanding of the links between ocean chemistry and the evolution of life on Earth.

氧气是维持地球上多种生命形式的关键。与今天不同，当生命最初进化时，大气和海洋基本上不含氧气。各种证据表明，大气中的氧含量在大约23亿年前才开始上升。直到最近，人们还认为这也导致了海洋的氧化（就像今天一样）。最近的一个假说认为，氧气的增加导致了陆地上硫化物矿物的风化，从而增加了河流向海洋输送的硫。然后海洋变得富含有毒的硫化氢，而不是氧气（现代黑海也有类似的情况）。事实上，海洋可能只是在大约7亿年前的第二次氧气上升之后才变得含氧。所有这些都对生物圈的进化产生了深远的影响。正是在海洋中，早期生命首先进化和繁荣。早期的生物圈是由细菌主导的，第一个光合细菌可能至少在27亿年前进化出来，在氧气第一次大幅度上升之前。早期生物圈的一个难题是，为什么产生氧气的光合作用的早期进化没有导致此后地球表面的快速氧化。同样清楚的是，高等生命形式，如植物和动物（最终是人类），只是在海洋最终变得含氧时才开始进化。为什么地球表面的氧化会延迟？为什么生物圈在地球历史的早期发展缓慢？最近一个著名的假说将古代地球的这些令人困惑的特征归因于海洋化学。光合细菌的关键需求之一是营养素，这些营养素基本上是肥料中含有的元素-磷，氮和微量金属，如钼（Mo）。在大气氧化之前，海洋可能富含溶解铁（溶于贫氧水中），导致富含铁的化学沉积物（所谓的带状铁地层或BIF）的广泛沉淀。它们可能带走了重要的营养物质，如磷和微量金属，留下非常低的浓度供细菌使用。在大气最初氧化之后，特别是如果海洋变成硫化物，微量金属也可能供应不足，因为其中许多金属在硫化氢存在的情况下沉淀下来。这一点很重要，因为它可能限制了光合作用，从而限制了氧气的产生，这有助于解释地球表面明显的延迟氧化，从而解释生物圈的缓慢进化。然而，这些想法仍然存在争议。需要进行详细的研究，以评估上述情况是否确实存在。同样重要的是要确定这些条件的普遍程度以及它们如何影响养分的可用性。该项目将研究生物多样性指数记录的营养物供应情况、大气中氧气首次上升后向富含硫化物的海洋过渡的全球范围、以及地球历史上随后导致动植物生命大爆发的时期海洋的化学演变。我们将使用的工具是钼的同位素。钼的海洋化学，特别是它从溶液中进入沉积物的过程，高度依赖于海洋的化学状态。此外，这些去除过程中有不同的偏好不同的同位素钼，这使得记录的钼同位素变化的岩石解释的古海洋的氧化状态，和钼作为营养物质的可用性。这项研究最终将使人们更好地了解海洋化学与地球生命进化之间的联系。